My mechanic once messed up and didn't mount a disc pad correctly, somehow it popped out and I was only using the one pad which didn't squeeze the disc as well as two would have. He was a bit red faced when I brought it back.
This was a one time thing but he does have a habit of setting my rim brakes in the open position then forgetting to unlock them after doing his work, I still have braking, just less of it till I realize whats happening and unlock them.